1. Field of Invention
The present invention relates to a method of driving a display device such that a gray-scale image is displayed by means of pulse width modulation with reduced electrical power consumption. The present invention also relates to a driver circuit based on such a method, a display device, and an electronic device.
2. Description of Related Art
In general, a portable electronic device includes a display device for presenting various kinds of information to a user. In display devices for such a purpose, information is displayed using an electrooptical change in an electrooptical material. For example, liquid crystal display devices are widely used for this purpose. In recent years, it has become desirable that display devices be capable not just of providing a simple on/off (two value) display but also of representing a large number of gray levels to display a gray-scale image.
However, in portable electronic devices which are driven by a battery, it is very important that the portable electronic devices operate with small power consumption. As is well known, a relatively greater power consumption is required to display a gray-scale image than is needed to provide a simple on/off display. That is, in display devices for use in portable electronic devices, it is necessary to meet both the requirements of having the capability of displaying a gray-scale image and having small power consumption, which seem to conflict with each other.
In view of the above, it is an object of the present invention to provide a method of driving a display device so as to display a gray-scale image without causing a significant increase in power consumption, a driver circuit for implementing the method and a display device using such a method, and an electronic device using such a display device.
According to a first aspect of the present invention, there is provided a method of driving a display device so as to display a gray-scale image by driving pixels disposed at locations corresponding to respective intersections of a plurality of scanning lines extending along rows and a plurality of data lines extending along columns. The method comprising the steps of sequentially selecting the plurality of scanning lines one by one every horizontal scanning period, and applying a selection voltage to a selected scanning line during one of two half periods that the horizontal scanning period has been divided into and applying a voltage to a pixel such that, in the case in which, of the plurality of scanning lines, a scanning line in either an odd-numbered or an even-numbered row is selected, a turn-on or a turn-off voltage is applied to a pixel at a location corresponding to an intersection of the selected scanning line and a data line belonging to a first group such that in one of the two half periods of each horizontal scanning period. The turn-on voltage is applied via the data line to the pixel during a period from the start of one of the two half periods of the horizontal scanning period until a time corresponding to a gray level has elapsed, and the turn-off voltage is applied to the pixel during the remaining period of said one of the two half periods. While in the case in which, of the plurality of scanning lines, a scanning line in the other one of odd-numbered and even-numbered rows is selected, the turn-on or the turn-off voltage is applied to the pixel at the location corresponding to the intersection of said scanning line and the data line belonging to the first group such that the turn-on voltage is applied via the data line to the pixel during a period from a point of time earlier than the end of the one of the two half periods of the horizontal scanning period by an amount of length of time corresponding to a gray level until the end of the one of the two half period, and the turn-off voltage is applied to the pixel during the remaining period of the one of the two half periods, and in either of these cases, in the other of said two half periods of the horizontal scanning period, the turn-on voltage is applied during a period equal to the period in which the turn-off voltage is applied in the one of the two half periods, while the turn-off voltage is applied during a period equal to the period in which the turn-on voltage is applied in the one of the two half periods.
In this first aspect of the present invention, for a pixel at a location corresponding to an intersection of a scanning line located in either an odd-numbered row or an even-numbered row and a data line belonging to the first group, a gray level is displayed using of left-side modulation. On the other hand, for a pixel at a location corresponding to an intersection of a scanning line located in the other one of the odd-numbered and even-numbered rows and a data line belonging to the first group, a gray level is displayed using of right-side modulation. This allows a reduction in the number of times the voltage applied to a data line is switched between the turn-on voltage and the turn-off voltage, and thus it is possible to reduce electric power consumed in switching the voltage.
In this first aspect of the present invention, if the plurality of data lines all belong to the first group, it is possible to achieve a simplified configuration because it is not necessary to distinguish data lines.
Furthermore, in the first aspect of the present invention, applying of voltage to a pixel may be performed such that in the case in which a scanning line in either an odd-numbered or an even-numbered row is selected, the turn-on or the turn-off voltage is applied to a pixel at a location corresponding to the intersection of the scanning line and a data line not belonging to the first group such that in one of the two half periods of each horizontal scanning period, the turn-on voltage is applied via the data line to the pixel during a period from a point of time earlier than the end of the one of the two half periods of the horizontal scanning period by an amount of length of time corresponding to a gray level until the end of the one of the two half periods, and the turn-off voltage is applied to the pixel during the remaining period of the one of the two half periods. While, in the case in which a scanning line in the other one of odd-numbered and even-numbered rows is selected, the turn-on or the turn-off voltage is applied to the pixel at the location corresponding to the intersection of the scanning line and the data line not belonging to the first group such that the turn-on voltage is applied via the data line to the pixel during a period from the start of one of the two half periods of the horizontal scanning period until a time corresponding to a gray level has elapsed, and the turn-off voltage is applied to the pixel during the remaining period of the one of the two half periods In either of these cases, in the other of the two half periods of the horizontal scanning period, the turn-on voltage is applied during a period equal to the period in which the turn-off voltage is applied in the one of the two half periods, while the turn-off voltage is applied during a period equal to the period in which the turn-on voltage is applied in the one of the two half periods. In this method, when a certain scanning line is selected, the timing of applying the turn-on voltage to a data line belonging to the first group and the timing of applying the turn-on voltage to a data line not belonging to the first group becomes different even for the same gray level. This allows a reduction in the number of data lines whose voltage is changed at the same time when the same gray level is displayed at a plurality of pixels. As a result, it becomes possible to suppress degradation in the selection voltage applied to a scanning line into a dull waveform, and thus it becomes possible to suppress degradation in image quality.
For the above reason, it is desirable that the number of data lines belonging to the first group be equal to or nearly equal to the number of data lines not belonging to the first group. In this case, even when the same intermediate gray level is displayed on all pixels at locations corresponding to a selected scanning line, the turn-on voltage supplied to data lines is switched only twice. Besides, the number of data lines which are switched at the same time becomes one-half the total number of data lines. More specifically, the data lines belonging to the first group may be those data lines, of the plurality of data lines, located in either odd-numbered columns or even-numbered columns.
According to a second aspect of the present invention, there is provided a driver circuit for driving a display device so as to display a gray-scale image by driving pixels disposed at locations corresponding to respective intersections of a plurality of scanning lines extending along rows and a plurality of data lines extending along columns. The driver circuit including a scanning line driver circuit for sequentially selecting the plurality of scanning lines one by one every horizontal scanning period, and applying a selection voltage to a selected scanning line during one of two half periods that the horizontal scanning period has been divided into and a data line driver circuit for applying a voltage to a pixel such that, in the case in which, of said plurality of scanning lines, a scanning line in either an odd-numbered or an even-numbered row is selected, a turn-on or a turn-off voltage is applied to a pixel at a location corresponding to an intersection of the scanning line and a data line belonging to a first group. The voltage is applied, such that in one of the two half periods of each horizontal scanning period, the turn-on voltage is applied via the data line to the pixel during a period from the start of one of the two half periods of the horizontal scanning period until a time corresponding to a gray level has elapsed, and the turn-off voltage is applied to the pixel during the remaining period of the one of the two half periods. While, in the case in which, of the plurality of scanning lines, a scanning line in the other one of odd-numbered and even-numbered rows is selected, the turn-on or the turn-off voltage is applied to the pixel at the location corresponding to the intersection of the scanning line and the data line belonging to the first group such that the turn-on voltage is applied via the data line to the pixel during a period from a point of time earlier than the end of the one of the two half periods of the horizontal scanning period by an amount of length of time corresponding to a gray level until the end of the one of the two half period, and the turn-off voltage is applied to the pixel during the remaining period of the one of the two half periods, and in either of these cases, in the other of said two half periods of the horizontal scanning period. The turn-on voltage is applied during a period equal to the period in which the turn-off voltage is applied in the one of the two half periods, while the turn-off voltage is applied during a period equal to the period in which the turn-on voltage is applied in the one of the two half periods. According to this second aspect of the present invention, as with the first aspect of the present invention, a reduction is achieved in the number of times the voltage applied to a data line is switched between the turn-on voltage and the turn-off voltage, and thus it is possible to reduce electric power consumed in switching the voltage.
According to a third aspect of the present invention, to achieve the above-described object, there is provided a display device having a pair of substrates, an electrooptical material disposed between the pair of substrates, and a plurality of scanning lines formed on one of the pair of substrates, a plurality of data lines formed on the other one of the pair of substrates. Pixels are disposed at locations corresponding to intersections of the plurality of scanning lines and the plurality of data lines. The display device serving to display a gray-scale image by driving the pixels. The display device further includes a scanning line driver circuit for sequentially selecting the plurality of scanning lines one by one every horizontal scanning period, and applying a selection voltage to a selected scanning line during one of two half periods that the horizontal scanning period has been divided into. The display device also includes a data line driving circuit for applying a voltage to a pixel such that in the case in which, of the plurality of scanning lines, a scanning line in either an odd-numbered or an even-numbered row is selected, a turn-on or a turn-off voltage is applied to a pixel at a location corresponding to an intersection of the scanning line and a data line belonging to a first group such that in one of the two half periods of each horizontal scanning period. The turn-on voltage is applied via the data line to the pixel during a period from the start of one of the two half periods of the horizontal scanning period until a time corresponding to a gray level has elapsed, and the turn-off voltage is applied to the pixel during the remaining period of the one of the two half periods. While, in the case in which, of the plurality of scanning lines, a scanning line in the other one of odd-numbered and even-numbered rows is selected, the turn-on or the turn-off voltage is applied to the pixel at the location corresponding to the intersection of the scanning line and the data line belonging to the first group such that the turn-on voltage is applied via the data line to the pixel during a period from a point of time earlier than the end of the one of the two half periods of the horizontal scanning period by an amount of length of time corresponding to a gray level until the end of the one of the two half periods. The turn-off voltage is applied to the pixel during the remaining period of the one of the two half periods, and in either of these cases, in the other of the two half periods of the horizontal scanning period, the turn-on voltage is applied during a period equal to the period in which the turn-off voltage is applied in the one of the two half periods, while the turn-off voltage is applied during a period equal to the period in which the turn-on voltage is applied in the one of the two half periods. According to this third aspect of the present invention, as with the second or the second aspect of the present invention, a reduction is achieved in the number of times the voltage applied to a data line is switched between the turn-on voltage and the turn-off voltage, and thus it is possible to reduce electric power consumed in switching the voltage.
In this third aspect of the present invention, preferably, the pixel includes a switching element and a capacitor formed of the electrooptical material, and the capacitor is driven by the switching element. In this construction, a selected pixel and a non-selected pixel is electrically isolated from each other by the switching element, and thus good contrast and response can be obtained and a high-quality image can be displayed.
In this construction, a thin film diode having a conductor/insulation/conductor structure may be preferably used as the switching element. In this case, one end of the thin film diode is connected to either a scanning line or a data line, and the other end thereof is connected to the capacitor. When the thin film diode is used as the switching element, the production process becomes simpler. Besides, principally, no short-circuited path is created between a scanning line and a data line.
According to a fourth aspect of the present invention, there is provided an electronic device including a display device according to the previous aspect of the invention. The electronic device according to the present invention is capable of displaying a gray-scale image with reduced power consumption.